In a statement to the media he insisted he did "not have any more secrets" to reveal but called on Israel to open up the Dimona nuclear plant where he worked to inspections by the UN's nuclear watchdog.

Answering only questions in English, Mr Vanunu said he was "not totally free" because of the restrictions Israel had placed on him, including a ban on travel outside the country.

Speaking briefly in Hebrew, he accused Israel of being a dictatorship and an apartheid country because of restrictions imposed on him preventing him speaking to foreigners.

"I tell the Israelis if you discriminate and prevent me from talking to citizens abroad then I am also not talking to you in Hebrew," he said.

He went on to say he had been treated harshly because of his conversion to Christianity.

Correspondents say Mr Vanunu's words seem sure to incense many Israelis who already view him as a traitor and a spy.

Restrictions

Despite his protestations, the authorities say Mr Vanunu still possesses information that could jeopardise Israel's security.

"He is a man sentenced to prison for treason and he has repeatedly said he will go back to his old behaviour... [Israel] has to take precautions to prevent that from happening," said Foreign Ministry spokesman Jonathan Peled.

Mr Vanunu is not allowed to have a passport, is forbidden to approach ports and airports, and has been told not to talk to foreigners without permission.

Mr Peled told the BBC that he was allowed to talk to the international media, as long as he did not talk about his work at the Dimona plant.

"We have to strike the correct balance between safeguarding his personal liberty and looking after Israel's national security interests," Mr Peled said.

On the basis of the information he gave to the UK's Sunday Times newspaper in 1986, analysts concluded Israel had scores of nuclear warheads.

Israel maintains a policy of "strategic ambiguity" about its supposed nuclear arsenal and it has refused to sign the Nuclear Non-Proliferation Treaty which would open Dimona up to international scrutiny.

Supporters had gathered outside the jail, waving banners and calling Vanunu a "hero of peace". Critics' banners had messages like "Death to the spy, Death to Vanunu".

Israel said it could have placed much tougher post-release restrictions on Mr Vanunu - and the length of time the current regime will remain in force depends on his behaviour.